WASHINGTON — The Food and Drug Administration announced Tuesday that it would scrap a decades-old lifetime prohibition on blood donation by gay and bisexual men, a major stride toward ending what many had seen as a national policy of discrimination.

However, the agency will continue to ban men who have had sex with a man in the last year, saying the barrier is necessary to keep the blood supply safe, a move that frustrated rights groups that were pushing for the ban to be removed entirely.